Large equipment such as high voltage circuit breakers or other switching devices, cannot respond adequately when exposed to a seismic disturbance with a frequency at or above the equipment's natural frequency. As a result of the seismic disturbance, resonant energy builds within the equipment. If the resonant energy becomes too large, deformities can result in the structural materials of the equipment. Ultimately, if a large number of structural deformities occur, the equipment may suffer catastrophic failure.
It is known to utilize spring deflection to absorb the energy of suddenly applied loads and store the energy for subsequent release. The amount of energy stored in a spring is a function of the spring's stiffness K or the spring constant and the distance x the spring is compressed. Some devices attempt to accommodate seismic deflections by either storing the kinetic energy of the seismic force and subsequently releasing the energy back to the earth, or by transferring the energy to the attached equipment, often at a lower frequency than the original seismic force. However, these devices when connected to the equipment have been ineffective in protecting the equipment from sudden seismic events, in part, because such devices do not effectively reduce the equipment's natural frequency.
Often, the spring constants are so large (i.e. the springs are so stiff), that the springs do not reduce the equipment's natural frequency when connected to the equipment in response to low frequency events like earthquakes. Rather, the seismic forces are transferred through the unreactive spring-loaded devices to the equipment. Ultimately, if the forces exerted through the spring are of a large enough magnitude and high enough frequency, the electrical equipment can fail. Therefore, the equipment's natural frequency should be lower with the damping devices than without the damping devices to prevent such failure.
FIG. 1 illustrates a typical arrangement of seismic devices. As shown, damping devices 60, 60' are commonly mounted on each of the support studs 40, 40' of the equipment which is shown generally at 14. A horizontal component 46h of the seismic force 46 that is exerted at the base of the equipment causes a downward vertical force 48 to be exerted on one support stud 40 of the equipment 14. A reactive upward force 50 is exerted on the opposite support stud 40' of the equipment 14. As one support stud 40 of the equipment 14 is forced down, and the opposite support stud 40' of the equipment 14 is forced up, the center of mass 44 of the equipment undergoes a horizontal seismic force 52. In those arrangements when the spring-loaded support device provides little or no damping, the equipment rebounds between the damping devices, imparting a "rocking" motion on the equipment 14. Each cycle of the "rocking," causes horizontal seismic forces 52 to be exerted on the equipment 14. The longer the "rocking" and related horizontal seismic forces 52 are exerted, the more resonant energy builds in the equipment.
By providing a damping capability in a seismic support device, the magnitude of horizontal seismic forces and their duration can be reduced. Several spring loaded seismic devices provide damping in a single direction, but none provide damping in response to both upward and downward forces. For example, some devices provide damping in response to a downward force but not in response to an upward force. Others provide damping only in response to upward forces and not downward forces. Using FIG. 1 as an example, if the pictured seismic devices 60, 60' both provided damping in response to downward forces, only one of the two seismic devices (i.e., the one receiving the downward force, 60) at any one time would provide damping as the equipment 14 rocked from side to side. Similarly, if both devices provide damping only in response to upward force, only one device (i.e., the seismic device receiving the upward force, 60') would actively damp at a given time as the equipment "rocks" from side to side. While such seismic support device may provide some damping in a single direction, such devices do not sufficiently reduce the magnitude and duration of the horizontal seismic forces that are exerted on the equipment. Thus the natural frequency of the equipment in response to a seismic disturbance is not reduced through the use of such devices.
Attempts have been made using single-action dampers to provide double acting damping. Often this has been accomplished through the coupling of two or more single acting dampers. Such arrangements have proven cumbersome because equipment are typically designed to be mounted to a single device. Generally there is no simple method of coupling more than one damper to the equipment at a time. Therefore, these arrangements have proven cumbersome and difficult to implement.
Other prior art seismic devices rely on hydraulic shock absorbers. Unlike the spring loaded devices, the hydraulic devices have been effective at damping the forces that are transferred to an attached equipment. However, these devices sometimes leak hydraulic fluid and have proven by field experience to be maintenance intensive.
Therefore, there is a need to provide an improved seismic damper that is capable of supporting large, heavy equipment, that reduces the natural frequency of equipment in response to seismic events through double acting damping, and can be easily installed without modification to the equipment.